The destruction of finger joints by a traumatism, degenerative or inflammatory rheumatism and, more rarely, an infection, creates a serious handicap which has been attempted to be overcome by implantation of metacarpophalangeal or interphalangeal prostheses.
For example, French Patent 2,242,067 describes a prosthesis made up of a piece of flexible material having transversely, in its middle, a thinned part which forms a flexible hinge. The ends of the piece constitute implant tails which are forced into the medullary canals.
Such prostheses allow a certain mobility to be recovered, but nevertheless impose a limitation on the mobility of the joint. In addition, numerous cases have been observed of osseous reactions around the prosthesis stems as a result of the wearing of the material and the emission of foreign bodies into the bone. The least serious disadvantage, which results from the repeated sliding of the stem in the medullary canal, is the possible enlargement of the latter.
French Patent 2,450,600 avoided this disadvantage by providing, on the joint components, pins capable of sliding freely in bushings formed in pads embedded in the medullary canals. The joint is thus held in place solely by the ligaments and tendons. The joint is made up of a head of cylindrical form mounted on the pin which slides in one of the medullary end pieces. The head cooperates with the concave surface of a plastic stem accommodated in a socket mounted on the pin sliding in the other end piece. The joint axis is the longitudinal axis of the cylindrical head. In order to achieve a certain degree of freedom in the radio-cubital plane of the articulation, the ends of the cylindrical head are hemispherical. The joint is held and returned by the natural ligaments and tendons.
In the hinge prosthesis, there are losses in extension and flexion due to the fact that the extensor and flexor tendons cannot ensure their function of adapting to the variable length of the fingers between the extension and flexion positions.
More particularly, if the length of the dorsal face of a finger is measured, it is found that the finger is shorter in extension than in flexion. This difference is explained by the fact that the phalanges in the flexion position put themselves one in front of the other (FIG. 1) and do not turn about a single axis. To each angle of flexion there corresponds an axis of rotation. The distal part of the metacarpals and also of the phalanges has, in the sagittal plane, a head which is asymmetrical relative to the axis of the metacarpal or phalanx projecting in the palmar direction.
The compensation mechanisms existing in the flexor tendons and the extensor tendons make it possible to redress the differences in length, for example, by bringing closer together or distancing the small lateral ligaments of the extensors in extension and in flexion (FIGS. 2A and 2B).